The present invention concerns a process for refining silver bullion. Silver bullion is a raw silver alloy originating amongst others from lead-silver smelting, containing generally more than 90% silver besides Se, Pb, Au, Cu and platinum group metals (PGM) as main impurities. Silver bullion is normally available as large castings.
The present state of technology for refining silver bullion alloy is electrolytic refining: after casting anodes, the alloy is electrorefined in an AgNO3xe2x80x94HNO3 electrolyte. The following output streams are hereby produced:
purified Ag obtained as dendrites deposited on a stainless steel cathode sheet,
anode slime, containing Au and PGM, which is collected in fabric bags surrounding the anode;
a bleed on the AgNO3xe2x80x94HNO3 electrolyte.
The electrolyte bleed is necessary to prevent the accumulation of impurities which anodically dissolve completely as nitrates, such as Pb and Cu, or partially, such as Pd.
A first method for bleed treatment is the retrieval of the dissolved Ag by cementation with a less noble metal. The impurities such as Pb and Cu remain in the nitrate bearing bleed solution.
A second method for bleed treatment is by denitration, also referred to as the black melt process in Ullmann""s Encyclopedia of Industrial Chemistry, 1993, vol.A24, p. 134. In this case, the water is evaporated from the electrolyte, and the anhydrous nitrate melt is heated to at least 170xc2x0 C. The nitrates of PGM decompose to insoluble oxides. Cu nitrate decomposes partially, to an extent determined by the temperature of the melt. After reaction with water, the oxides are separated from the AgNO3 bearing solution. However, if an excess of Pb and Cu has to be removed from the electrolyte, at least part of the bleed must be treated according to the first method. Here again, the Ag has to be retrieved by cementation, producing an impure Pb bearing nitrate solution.
Both methods thus necessitate a rather elaborate treatment of the bleed whereby nitrate solutions are produced. Their further treatment leads to the discharge of nitrates.
This disadvantage is further exacerbated when higher levels of Pb, Cu or Pd are present in the Ag bullion: a higher amount of impurities has indeed to be evacuated by bleeding a correspondingly larger quantity of electrolyte.
Also, more than approximately 2% Pd in the raw silver leads to problems as Pd then gets embedded in the refined Ag deposit.
Both the electrolytic refining process and the anode slime treatment are inherently slow. Consequently, metals have a long residence time and the inventory of the refining plant is high as are the ensuing financial costs.
The present invention aims at resolving the above mentioned disadvantages. Moreover, the new process produces Ag with a higher purity than the state of the art process.
It should be noted that JP-A-60224720 discloses a process for the recovery of Ag from Cu-electrolysis anode slime. This process presupposes that Cu has been nearly completely removed from the slime and that a crude metallic Ag is produced. This crude Ag is subjected to a melting step with injection of an O2 carrying gas, ensuring the oxidation and removal of impurities. The molten purified Ag is then granulated and the granules are dissolved in HNO3. The obtained solution is cleaned up using a chelating resin, whereupon Ag is recovered from the solution by reduction with hydrazine.
The presupposed removal of Cu is however a complex and lengthy process as it necessitates the chlorination (wet or dry) of the anode slime, followed by the conversion of the chlorides back to their metallic form.
U.S. Pat. No. 5,000,928 describes a process for the preparation of ultra-pure AgNO3. As a first step, crude Ag is dissolved in HNO3. It is disclosed that heating and aerating promotes the dissolution process. Further steps include the addition of an alkaline agent to precipitate impurities and the use of a selective reducing agent to precipitate Ag as a metallic powder. This powder is then again dissolved with HNO3, whereupon ultra-pure AgNO3 is crystallized from the solution.